In a heavy 10-inch straight-sided sauté pan set over medium- low heat, slowly cook the pancetta in 1 Tbs. of the oil until golden and crisp all over, 10 to 15 minutes.

With a slotted spoon, transfer the pancetta to a plate lined with paper towels, leaving the fat behind. You should have about 2 Tbs. of fat in the pan; if not, add the remaining 1 Tbs. oil. Have ready 1/2 cup water. Put the pan over medium-high heat and arrange the sprouts cut side down in a single layer. Cook undisturbed until nicely browned, 2 to 3 minutes. When the sprouts are browned, add the water to the pan, cover immediately, and simmer until the sprouts are tender when poked with a fork or skewer, about 3 minutes. (If the water evaporates before the sprouts get tender, add more water, 1/4 cup at a time.) With a slotted spoon, transfer the sprouts to a plate.

Return the pan to medium-high heat and if any water remains, let it boil off. Add the balsamic vinegar and a few grinds of pepper. Boil the vinegar until it’s reduced to about 2 Tbs. and looks lightly syrupy, about 2 minutes. Reduce the heat to low, add the butter, and stir until melted. Return the sprouts and pancetta to the pan and swirl and shake the pan to evenly coat the sprouts with the sauce. Season to taste with salt and more pepper and serve.